Magnetic resonance system

ABSTRACT

A magnetic resonance system has a primary RF transmitting antenna and at least one secondary RF transmitting antenna. Both antennas generate RF excitation fields in an examination volume. At least one of the primary and secondary transmitting antennas has a control element connected thereto, which allows an amplitude ratio and/or a phase difference of the respective excitation currents in the antennas to be set by a control and evaluation device. The control and evaluation device causes the excitation current in the secondary transmitting antenna to have a smaller amplitude than the excitation current in the primary transmitting antenna, so that the excitation field generated by the secondary transmitting antenna has a smaller spatial extent in the examination volume than the excitation field generated by the primary transmitting antenna.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention concerns a magnetic resonance system having abasic field magnet, a gradient system, a primary RF transmittingantenna, at least one secondary RF transmitting antenna, at least one RFreceiving antenna and a control and evaluation device for operating thegradient system and the antennas and for evaluating he signalstherefrom.

2. Description of the Prior Art

Magnetic resonance systems are generally known. The text “BildgebendeSysteme für die medizinische Diagnostik”, 3rd edition, 1995, PublicisMCD Verlag, pages 501 through 503, is referenced as an example of ageneral description of such systems.

The known resonance systems exhibit a basic magnetic field that isnormally 1.5 T. A good reconstruction of the examination subject ispossible with this magnetic resonance system.

In earlier times, magnetic resonance systems were also produced in whichthe basic magnetic field exhibited more than 1.5 T, in particular 3 Tand more. Better resolutions are achievable with these magneticresonance systems. However, image inhomogeneities that are caused byradio-frequency eddy currents in the examination subject increasinglyoccur with such magnetic resonance systems in the prior art.

In the prior art—, for example, U.S. Pat. No. 6,252,403—it is proposedto fashion the transmitting antenna spiral-shaped to compensate sucheddy currents. The transmitting antenna thus assumes the shape of abirdcage resonator wound around its axis of symmetry. It is also knownfrom this patent to arrange suitable dielectrics, in particular water,near the examination subject in order to hereby achieve a homogenizationof the radio-frequency excitation field. In spite of such endeavors,however, inhomogeneities in the excitation field cannot be sufficientlycompensated in all cases.

A magnetic resonance system according to the preamble of the claim 1 isknown from U.S. Pat. No. 4,682,112 that has at least one control elementthat allows setting or adjustment of the amplitude ratio and/or phaseoffset of the respective excitation currents in the primary andsecondary RF antennas. In this magnetic resonance system, thetransmitter arrangement is comprised of an array of identicallyfashioned transmitter antennas that are separated axially and/or in thecircumferential direction from one another. They can be individuallyactivated by the control and evaluation device.

A magnetic resonance system having a number of receiving antennas thatcan be individually activated is known from U.S. Pat. No. 5,216,367.

A magnetic resonance system in which the transmitting antenna isfashioned as a modified birdcage resonator is known from U.S. Pat. No.5,642,048.

From the scientific paper “Reduction of RF Penetration Effects in HighField Imaging” by Thomas K. F. Foo, Cecil E. Hayes and Yoon-Won Kang,appearing in Magnetic Resonance in Medicine 23 (1992), issue 2, pages287 through 301, it is known to change the dielectric coupling of theradio-frequency shield of a whole-body transmitting antenna in order toachieve an optimally homogenous radio-frequency field within anexcitation volume.

SUMMARY OF THE INVENTION

An object of the present invention is to achieve a simply fashionedmagnetic resonance system in which the excitation field is optimallyhomogenous in spite of a strong basic magnetic field in the examinationsubject.

The object is achieved by a magnetic resonance system of the typeinitially described, also having at least one control element asdescribed above, wherein the control and evaluation unit causes theexcitation current flowing in the secondary transmitting antenna toexhibit a smaller amplitude than the excitation current flowing in theprimary transmitting antenna, and wherein the excitation field generatedby the secondary transmitting antenna exhibits a smaller spatial extentthan the excitation field generated by the primary transmitting antenna.

The secondary transmitting antenna thus preferably acts only locally forthe correction of the excitation field generated by the primarytransmitting antenna. By this design, the excitation field generated bythe primary transmitting antenna can be specifically and activelyhomogenized. Thus the (at least one) control element is activated suchthat the superimposition of the excitation fields is optimallyhomogenous in the examination subject given an examination subjectintroduced into the examination region.

It is possible for the secondary transmitting antenna to be operatedcompletely independent of the primary transmitting antenna. Preferably,however, it is inductively coupled with the primary transmittingantenna.

A control signal, on which the activation of the control element isdependent, can be provided to the control and evaluation device,allowing an adaptation to the examination subject. The control signal,for example, can be implicitly determined by body size and weight(possibly also further characteristics) of a patient to be examined. Theactivation alternatively can be determined by tests.

In individual cases, the secondary transmitting antenna can also beoperated dependent on the control signal, such that it is not resonantat the magnetic resonance frequency that is being employed and does notcharge the examination region with an excitation field. The secondarytransmitting antenna thus also can be completely deactivated in theindividual case.

The control element is fashioned as an adjustable impedance, allowingthe amplitude ratio and/or phase offset of the excitation currents to beparticularly flexibly adjusted.

The excitation current flowing in the secondary transmitting antenna canexhibit a phase offset between −180° and +180° relative to theexcitation current flowing in the primary transmitting antenna.

The secondary transmitting antenna can be connected with the control andevaluation device such that it cannot be used for the reception of themagnetic resonance signals. In this case, the secondary transmittingantenna is thus exclusively used for the homogenization of theexcitation field. However, it is also possible for the secondarytransmitting antenna also to be used for reception of the magneticresonance signals. In this case it is preferably fashioned as a localcoil of the type typically used for the local reception of magneticresonance signals. In particular in this case, upon reception ofmagnetic resonance signals the secondary transmitting antenna can betuned by the control and evaluation device such that it is resonant atthe employed magnetic resonance frequency.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a magnetic resonance system constructed inaccordance with the principles of the present invention.

FIG. 2 illustrates a primary transmitting antenna and a secondarytransmitting antenna in accordance with the invention.

FIG. 3 illustrates another embodiment of a secondary transmittingantenna in accordance with the invention.

FIG. 4 illustrates a further embodiment of a primary transmittingantenna and a secondary transmitting antenna in accordance with theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1, a magnetic resonance system has an examinationvolume 1. An examination subject, here a person 3, can be introducedinto the examination volume 1 by means of a patient bed 2.

The examination volume 1 is charged with a basic magnetic field by meansof a basic field magnet 4. The base magnetic field is temporallyconstant (static) and spatially as homogenous as possible. It exhibits amagnetic field strength that is greater than 1.5 T, preferably 3 T ormore.

The basic field magnet 4 is preferably is a superconducting magnet. Nofurther activation and/or linkage via the control and evaluation device5 is thus necessary.

The magnetic resonance system also has a gradient system 6 by means ofwhich the examination volume 1 can be charged with gradient magneticfields. The gradient system 6 can be activated by the control andevaluation device 5 such that gradient currents flow in the gradientsystem 6.

The magnetic resonance system also has a primary antenna 7 and asecondary antenna 8. The primary antenna 7 is fashioned as a whole-bodyantenna. In contrast, the secondary antenna 8 is fashioned as a localcoil. It thus acts only locally. The antennas 7, 8 can be activated bythe control and evaluation device 5 such that excitation currents flowin the antennas 7, 8.

The examination region 1 can be charged with a radio-frequencyexcitation field by means of the primary antenna 7. When the examinationsubject 3 is located in the examination volume 1, the examinationsubject 3 can thus be excited to magnetic resonances. The magneticresonance signals thus generated are then alternatively receivable bymeans of the primary antenna 7 or the secondary antenna 8. Both antennas7, 8 thus represent receiving antennas 7, 8 of the magnetic resonancesystem. The received magnetic resonance signals can be supplied to thecontrol and evaluation device 5 and evaluated therein in a known manner.

Due to the high basic magnetic field of 3 T and more, it s not possiblein all cases to generate a homogenous excitation field in the entireexamination subject 3 by means of only the primary antenna 7. Thereforethe secondary antenna 8 can also be activated by the control andevaluation device 5 such that it likewise at least locally charges theexamination subject 3 with a radio-frequency excitation field. However,the excitation field generated by the secondary antenna 8 exhibits asmaller spatial extent than the excitation field generated by theprimary antenna 7. The excitation current flowing in the secondaryantenna 8 also exhibits a smaller amplitude than the excitation currentflowing in the primary antenna 7.

As shown in FIG. 2, the secondary antenna 8 can be activated by thecontrol and evaluation device 5 independent of the primary transmittingantenna 7. The control and evaluation device 5 thus itself represents acontrol element by means of which an amplitude ratio and/or a phaseoffset of the excitation currents flowing in the antennas 7, 8 can beadjusted.

The activation of the secondary antenna 8 can ensue via the control andevaluation device 5 such that the phase offset is between −180° and+180°. It is thus arbitrarily adjustable. The amplitude ratio isalso—within the design limits of the secondary antenna 8—arbitrarilyadjustable. In particular the secondary antenna 8 can be operated in theindividual case such that it does not charge the examination region 1with an excitation field. In this case the excitation current flowing inthe secondary antenna 8 thus has the value 0.

A control element 9 is arranged in the secondary antenna 8. If it isintended that the secondary antenna 8 should not charge the examinationregion 1 with an excitation field, the secondary antenna 8 is detuned bymeans of the control element 9 such that it is not resonant at themagnetic resonance frequency.

The activation of the secondary antenna 8 ensues dependent on a controlsignal S that is externally predetermined for the control and evaluationdevice 5 by a user. The activation of the control element 9 is thusdependent on the control signal S.

According to FIG. 3, the control element 9 exhibits a number ofcapacitors 10 and a parallel oscillating circuit 11. One of thecapacitors 10 or the parallel oscillating circuit 11 is switched on viaa switch 12, dependent on the control signal S. The control element 9 isthus fashioned as an adjustable impedance 9.

Upon activation of one of the capacitors 10—depending on the connectedcapacitor 10—an increase or an attenuation of the excitation fieldgenerated by the primary transmitting antenna 7 or an exact tuning ofthe secondary antenna 8 to the magnetic resonance frequency ensues. Thetuning to the magnetic resonance frequency is only effected in the eventof reception.

If the parallel oscillating circuit 11 is connected, the secondaryantenna 8 is detuned such that it can no longer be excited in themagnetic resonance frequency. This corresponds to a deactivation of thesecondary antenna 8 when this should thus not be used at all in thetransmitting and receiving case.

The representation according to FIG. 4 generally corresponds to that ofFIG but, in FIG. 4 no separate excitation of the secondary antenna 8ensues. Rather, an inductive coupling M exists between the antennas 7,8. The secondary antenna 8 is thus inductively coupled with the primaryantenna 7. In this case, not only does an adjustment of the resonancefrequency of the secondary antenna 8 ensue by means of the controlelement 9, but rather with the adjustment of the resonance frequency anadjustment of amplitude and phase offset of the excitation currentflowing in the secondary antenna 8 ensues relative to the excitationcurrent flowing in the primary antenna 7.

Both in the embodiment according to FIG. 2 and in the embodimentaccording to FIG. 4, the activation of the control element 9and—directly or indirectly—the secondary antenna 8 ensues such that thesuperimposition of the excitation fields is optimally homogenous in theexamination subject 3.

When receipt of magnetic resonance signals by the secondary antenna 8 isdesired, it is always tuned by the control and evaluation device 5 suchthat it is resonant at the magnetic resonance frequency.

In the above it has been specified that the secondary antenna 8 is usedboth to charge the examination region 1 with an excitation field and toreceive magnetic resonance signals. Naturally it is also possible to usethe secondary antenna 8 exclusively to charge the examination region 1with an excitation field. In this case the secondary antenna 8 is thusnot used to receive magnetic resonance signals.

In spite of the high basic magnetic field, a nearly completelyhomogenous excitation field is thus realizable in a simple manner bymeans of the inventive magnetic resonance examination system.

Although modifications and changes may be suggested by those skilled inthe art, it is the invention of the inventors to embody within thepatent warranted heron all changes and modifications as reasonably andproperly come within the scope of their contribution to the art.

1. A magnetic resonance system comprising: a basic field magnet thatgenerates a homogenous, static basic magnetic field in an examinationvolume; a gradient system operable to superimpose at least one gradientmagnetic field on said basic magnetic field in said examination volume;a primary RF transmitting antenna operable with an excitation current togenerate an RF excitation field in said examination volume; a secondaryRF transmitting antenna operable with an excitation current to generatean RF excitation field in said examination volume; an RF receivingantenna operable to receive magnetic resonance signals from saidexamination volume produced by magnetic resonance in an examinationsubject adapted to be disposed in said examination volume; a controlelement electrically connected to one of said primary and secondary RFtransmitting antennas allowing setting of at least one of an amplituderatio and a phase offset of the respective excitation currents in theprimary and secondary transmitting antennas; and a control andevaluation unit connected to said gradient system, said primary andsecondary RF transmitting antennas, and said RF receiving antenna, foroperating said gradient system to generate said gradient magnetic fieldsand for operating said primary and secondary RF transmitting antennas,and said at least one control element, to generate the respectiveexcitation currents in the primary and secondary RF transmittingantennas, and for evaluating the magnetic resonance signals received bysaid RF receiving antenna, said control and evaluation unit causing theexcitation current in the secondary RF transmitting antenna to have asmaller amplitude than the excitation current in the primary RFtransmitting antenna, for causing the RF excitation field generated bythe secondary RF transmitting antenna to exhibit a smaller spatialextent than the RF excitation field generated by the primary RFtransmitting antenna.
 2. A magnetic resonance system as claimed in claim1 wherein said control and evaluation unit activates said at least onecontrol element for causing a superimposition of the respective RFexcitations fields in the examination subject in the examination volumeto be substantially homogenous.
 3. A magnetic resonance system asclaimed in claim 1 wherein said secondary RF transmitting antenna isinductively coupled with said primary RF transmitting antenna.
 4. Amagnetic resonance system as claimed in claim 1 wherein said control andevaluation unit generates a control signal to said at least one controlelement for activating said at least one control element dependent onsaid control signal.
 5. A magnetic resonance system as claimed in claim4 wherein said control and evaluation unit also supplies said controlsignal to said secondary RF transmitting antenna, for causing saidsecondary RF transmitting antenna not to be resonant at a magneticresonance frequency employed for generating said magnetic resonancesignals in the examination subject, so that said examination subject isnot charged with an RF excitation field from said secondary RFtransmitting antenna.
 6. A magnetic resonance system as claimed in claim1 wherein said control element comprises an adjustable impedance.
 7. Amagnetic resonance system as claimed in claim 1 wherein said control andevaluation unit controls the excitation current in said secondary RFtransmitting antenna to have a phase offset between −180° and +180°relative to the excitation current in the primary RF transmittingantenna.
 8. A magnetic resonance system as claimed in claim 1 whereinsaid secondary RF transmitting antenna is operated by said control andevaluation unit to be insensitive for receiving said magnetic resonancesignals.
 9. A magnetic resonance system as claimed in claim 1 whereinsaid control and evaluation unit operates said secondary RF transmittingantenna to serve as said RF receiving antenna.
 10. A magnetic resonancesystem as claimed in claim 9 wherein said control and evaluation unitoperates said secondary RF transmitting antenna to serve as said RFreceiving antenna by tuning said secondary RF transmitting antenna to beresonant at a magnetic resonance frequency used to produce said magneticresonance signals.